The present invention relates to optical scopes such as borescopes and endoscopes which include a system for measuring the dimensions of a remote or inaccessible feature which is viewed through the scope.
An example of this type of scope is described in International Patent Application WO96/20389. A typical application for such scopes is in the inspection of gas turbine engines. The scope can be used to inspect the condition of components such as turbine blades to see whether there are any faults which require the engine to be stripped down in order to access the blades for repair or replacement. Systems which allow a feature viewed through the scope to be measured have been developed so that the seriousness of any faults which are seen can be assessed. If the size of the fault is within a particular range, it may be unnecessary to repair or replace the blade and an engine strip down would not therefore be required. Thus, scopes which enable dimensions of features viewed through them to be accurately measured can dramatically reduce the time and cost of operations such as engine inspection.
The system described in WO96/20389 determines the dimensions of a feature on the basis of the distance of the feature from the scope (the object distance) and the magnification of the scope. The object distance is determined by means of the focus control. An image of the feature is brought into sharp focus and a high resolution encoder associated with the focus control is used to determine the focus position. A processor then converts the focus position data into an object distance signal.
A problem with this system is that it is subjective because different operators may consider that the image is in focus at slightly different positions of the focus control. This leads to inaccuracy and inconsistency in determination of the object distance.
A number of systems currently exist which use the principals of triangulation to measure features and contours on an item. For example, European patent 0442393 uses a pair of lasers to measure by triangulation the contours of vehicle body panels. Similarly, U.S. Pat. No. 4,325,640 uses laser triangulation for measuring the contours of helicopter rotor blades. However, such systems rely on the workpiece to be measured being readily accessible so that the measuring system can be moved freely around it.
The present invention provides an optical scope for viewing and determining a dimension of a feature at an inaccessible location, comprising a tube having a distal end which is insertable in use into an inaccessible location and a proximal end connected to a housing; an optical system for gathering an image of a feature and relaying the image to a viewing means; means to direct a laser beam to a first reference point in the distal end of the cube; a first reflector mounted in the distal end of the tube for rotation about the first reference point and being operable to change the direction of the laser beam so as to direct the beam out of the scope to a target feature; a second reflector mounted in the distal end of the tube for rotation about a second reference point and being operable to change the direction of a reference axis intersecting the second reference point so as to cause the axis to intersect the laser beam at the target feature; control means actuable at the proximal end of the scope to rotate the first and second reflectors about the first and second reference points; position sensing means to detect movement of the first and second reflectors, and antibacklash means associated with the control means.
Preferably, the control means for each reflector comprises a mechanical linkage extending between the reflector and a rotatable control collar mounted on the housing.
The mechanical linkage preferably comprises a pullwire.
Conveniently, the antibacklash means comprises spring means biasing each reflector in opposition to the control means.
The antibacklash means further may comprise a friction braked wheel to which the proximal end of the pullwire is secured.
Alternatively, the antibacklash means may further comprise a recirculating ball screw device to which the proximal end of the pullwire is secured.
In another alternative, the antibacklash means further comprises a first cylinder located, and constrained to move axially, in the housing and to which the proximal end of the pullwire is secured, a second cylinder located proximally of the first cylinder and also constrained to move axially, spring means urging the first and second cylinders towards each other, a rotatable actuating collar linked to the first and second cylinders to move the cylinders axially upon rotation of the collar and spring means urging the collar axially against a fixed part of the housing.
In yet another antibacklash system, the pullwire is loosed around a wheel, whose axis is spring biased in the proximal direction, to provide two pullwire limbs, the first limb is attached to the reflector on one side of its pivot point, the second limb is attached to the reflector on the other side of its pivot point and first and second position sensing means are associated with the first and second limbs respectively.
In a preferred embodiment, the optical scope further comprises means to direct a second laser beam to the second reference point, whereby the reference axis is defined by the second laser beam.
In another embodiment, the scope has a viewing axis intersecting the second reference point whereby the reference axis is defined by the viewing axis.
In this case, a graticule may be mounted in the image transmission system, whereby when an image of a feature is aligned with the centre of the graticule the viewing axis is positioned so as to intersect the feature.
Preferably, the means to direct the first laser beam to the first reference point comprises at least one optical fibre and may further comprise at least one reflector mounted in the path of the beam.
Preferably, the means to direct the second laser beam to the second reference point comprises at least one optical fibre and may further comprise at least one reflector mounted in the path of the beam.
Conveniently, the position sensing means is located in the housing and may comprise a linear voltage displacement transducer, a Moire fringe device, a variable resistor potentiometer, a variable reluctance transformer, or an inductosyn.
In another aspect, the invention provides apparatus for determining a dimension of a feature at an inaccessible location, comprising an optical scope of the aforementioned type, viewing means for receiving an image relayed by the optical system of the optical scope and processing means for receiving data from the position sensing means and calculating a dimension of the feature from the data.
The viewing means preferably comprises a camera and a monitor for displaying the image received by the camera.